1. Field of the Invention
This invention relates to a mixture of stereoisomers with higher activity of an .alpha.-cyano-3-(4-halogenophenoxy) benzyl 2-(4-chlorophenyl)isovalerate of the formula (I): ##STR1## wherein X is a fluorine atom, a chlorine atom or a bromine atom, and * indicates an asymmetric carbon atom, which contains at least 60% of an enantiomer pair of a compound of the formula (I) having an (S)-configuration on both the acid and alcohol moieties and its enantiomer, or which consists substantially of the enantiomer pair, a process for preparing the same, and an insecticidal and/or acaricidal composition containing the same.
2. Description of the Prior Art
.alpha.-Cyano-3-(4-hologenophenoxy)benzyl 2-(4-chlorophenyl) isovalerates of the formula (I) are compounds useful as insecticides and acaricides. These esters have one asymmetric carbon atom on each of the acid and alcohol moieties. An ester provided by the conventional method is a mixture comprising substantially equal amounts of four isomers.
These optical isomers are hereunder referred to as shown in Table 1 below.
TABLE 1 ______________________________________ Abbreviations for Optical Isomers Acid Moiety (S)-Con- (R)-Con- Alcohol Moiety figuration Racemic figuration ______________________________________ (S)-Configuration A.alpha.-Isomer .alpha.-Isomer B.alpha.-Isomer Racemic A-Isomer "Racemate" B-Isomer (R)-Configuration A.beta.-Isomer .beta.-Isomer B.beta.-Isomer ______________________________________
The enantiomer pair of the A.alpha.-isomer and the B.beta.-isomer is referred to as a Y-isomer, and the enantiomer pair of the A.beta.-isomer and the B.alpha.-isomer as an X-isomer.
The A.alpha.-isomer, B.alpha.-isomer, A.beta.-isomer and B.beta.-isomer exhibit greatly differing insecticidal and/or acaricidal activity, and the A.alpha.-isomer, i.e., an ester having an (S)-configuration on both the acid and alcohol moieties, has the highest activity. An ester, "racemate", of the formula (I) synthesized from a conventional starting racemate contains almost equal amounts of all of the isomers, and therefore its activity is only about a quarter of the activity of the A.alpha.-isomer. For this reason, the method of producing an ester of the formula (I) with a high content of A.alpha.-isomer would be greatly advantageous in practice.
Possible means for achieving this purpose include one for obtaining an ester (A-isomer) having an (S)-configuration on the acid moiety (i.e., a mixture of the A.alpha.-isomer and the A.beta.-isomer), an ester having an (S)-configuration on the alcohol moiety (i.e., a mixture of the A.alpha.-isomer and the B.alpha.-isomer), or the A.alpha.-isomer per se. However, none of these techniques can be performed without an optically active starting material that is generally obtained by complicated procedures, i.e., optical resolution.
Combinations of the A.alpha.-isomer and the B.beta.-isomer, and the A.beta.-isomer and the B.alpha.-isomer are each in a relationship of enantiomer pair. The other combinations of isomers are diastereomers for one another. Therefore, if some means were devised that can provide a Y-isomer, it is expected that the insecticidal and/or acaricidal activity of an ester of the formula (I) in the racemic form obtainable by the conventional method will be almost doubled because the Y-isomer contains as much as 50% of an A.alpha.-isomer.
Heretofore, diastereomers have been isolated by chromatographic techniques such as column chromatography, thin-layer chromatography and gas chromatography. A Y-isomer or an X-isomer can be separated from the ester of the formula (I) in the racemic form. However, although separation by these chromatographic techniques may be feasible in a laboratory, they are virtually impossible to carry out on an industrial scale.